Third generation partnership project (3GPP) and Long Term Evolution (LTE) mobile telecommunication systems provide high data rate, lower latency and improved system performances. With the rapid development of “Internet of Things” (IOT) and other new user equipment (UE), the demand for supporting machine communications increases exponentially. To meet the demand of this exponential increase in communications, additional spectrum (i.e. radio frequency spectrum) is needed. The amount of licensed spectrum is limited. Therefore, communications providers need to look to unlicensed spectrum to meet the exponential increase in communication demand. One suggested solution is to use a combination of licensed spectrum and unlicensed spectrum. This solution is referred to as “Licensed Assisted Access” or “LAA”. In such a solution, an established communication protocol such as Long Term Evolution (LTE) can be used over the licensed spectrum to provide a first communication link, and LTE can also be used over the unlicensed spectrum to provide a second communication link.
Furthermore, while LAA only utilizes the unlicensed spectrum to boost downlink through a process of carrier aggregation, enhanced LAA (eLAA) allows uplink streams to take advantage of the 5 GHz unlicensed band as well. Although eLAA is straightforward in theory, practical usage of eLAA while complying with various government regulations regarding the usage of unlicensed spectrum is not so straightforward. Moreover, maintaining reliable communication over a secondary unlicensed link requires improved techniques.
In 3GPP Long-Term Evolution (LTE) networks, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, e.g., evolved Node-Bs (eNBs) communicating with a plurality of mobile stations referred as user equipment (UEs). Orthogonal Frequency Division Multiple Access (OFDMA) has been selected for LTE downlink (DL) radio access scheme due to its robustness to multipath fading, higher spectral efficiency, and bandwidth scalability. Multiple access in the downlink is achieved by assigning different sub-bands (i.e., groups of subcarriers, denoted as resource blocks (RBs)) of the system bandwidth to individual users based on their existing channel condition. In LTE networks, Physical Downlink Control Channel (PDCCH) is used for downlink scheduling. Physical Downlink Shared Channel (PDSCH) is used for downlink data. Similarly, Physical Uplink Control Channel (PUCCH) is used for carrying uplink control information. Physical Uplink Shared Channel (PUSCH) is used for uplink data. In addition, physical random access channel (PRACH) is used for non-contention based RACH on an eLAA carrier.
In some countries, there are requirements on the occupied channel bandwidth for unlicensed carrier access. Specifically, the occupied channel bandwidth shall be between 80% and 100% of the declared nominal channel bandwidth. During an established communication, a device is allowed to operate temporarily in a mode where its occupied channel bandwidth may be reduced to as low as 40% of is nominal channel bandwidth with a minimum of 4 MHz. The occupied bandwidth is defined as the bandwidth containing 99% of the power of the signal. The nominal channel bandwidth is the widest band of frequencies inclusive of guard bands assigned to a single carrier (at least 5 MHz).
In LTE, various PRACH formats are defined, including PRACH formats 0-3 and PRACH format 4. In all cases, however, PRACH occupies contiguous frequency tones and takes roughly 1.08 MHz, which does not meet Europe's regulation on occupied channel bandwidth. Therefore, a PRACH waveform design to satisfy the requirements on the occupied channel bandwidth in eLAA wireless communications network is sought.
Furthermore, traditional PRACH resource in LTE is allocated periodically and statically. In eLAA, however, any downlink and uplink access has to follow a successful listen-before-talk (LBT) channel access procedure. As a result, a successful PRACH transmission happens only when the LBT for a downlink scheduler for the PRACH is successful as well as the LBT for PRACH itself is also successful at the allocated time. Since the success of LBT cannot be guaranteed, the PRACH transmission thus cannot be guaranteed. Therefore, a method of PRACH resource allocation is sought to facilitate uplink random access in unlicensed carriers.